Tough question to answer. You will get several responses I'm sure . . . =)
IMHO, Myst is a classic and worth playing for the experience. It's not as beautiful as I remembered it but still has some amazing graphics especially as you move forward in the series. I have fond memories of playing the game as there was nothing like it when released. With so many Myst clones available you may not find it as entertaining today.
Myst Masterpiece Edition and Real Myst are the same except Real Myst is 3D. The 3D graphics are not quite as good as in the ME version but you can move around freely. Some say Real Myst is the best and some say Myst ME. I would read the reviews on both and start with the one that appeals to you. My pick would be Real Myst.
Riven was my favorite and exploring in Uru is more fun than playing the game. There are some frustrating puzzles but . . . there are many walkthrus available online that eliminate that problem. I'm not much for adventure games but I played through Syberia I and II because I wanted to play something like Myst again. I would say if you enjoyed Syberia you will enjoy the Myst series which, IMO, is a bit more of an adult story.
The order to play, if you want the whole story, is [ Myst ME or Real Myst ], Riven, Uru. I would recommend Riven if you just want to play the game.
There's my two cents . . . =)